Once an elementary school rite of passage, chicken pox (varicella) is much rarer these days, thanks to childhood immunization. But it's not yet a disease of the past. Here's the lowdown on those pesky pox.

Until quite recently, varicella (also known as chicken pox) was so common that there were as many cases per year in the U.S. as there were babies born – about 4 million. In fact, if you are old enough have an interest in reading this, you probably had chicken pox yourself. But thanks to a vaccine that came to the U.S. market in the mid '90s, it is a changing illness, and, with any luck, will soon be a rare one.

What it is:

Varicella is the disease caused by the varicella zoster virus (VZV). This virus belongs to the herpes virus family and, like the other herpes viruses, infection is lifelong. Exposure occurs through direct contact with pox lesions, or even more commonly, when infected droplets from one person's cough or sneeze enter the body of another through the respiratory tract. After an initial infection (the outbreak of chicken pox) the virus travels up the nerve roots where it lives in a dormant state. Once reactivated, years or decades later, it travels back down the sensory nerve root where it appears as a localized rash called zoster, or shingles. Most commonly, shingles afflicts an older person as his or her immunity weakens, but healthy children can get shingles as well.